It's the latest such development in the Arabian peninsula: Dubai opened the first subway system in the Gulf back in 2009, while Qatar has commissioned a metro to be built in Doha ahead of the 2022 World Cup.

Work will begin next year and is due to be completed by 2019. The planners expect that more than a million people a day will use the metro when it first opens, rising to 3.6 million after 10 years.

Saudi rulers hope the new system will cut Riyadh's traffic by encouraging Saudis to leave their gas-guzzling cars at home, and reducing pollution. They also want to encourage investment in the city, especially in sectors outside the oil industry.

But the biggest impact for ordinary residents of Riyadh is likely to be social.

Ibrahim al-Sultan, the official supervising the project, told Reuters that the metro will "enhance the quality of life" of Riyadh's population of nearly 6 million.

Among those most likely to benefit are the poor, and women — who are currently forbidden to drive cars, and instead must rely on taxis or a male relative to ferry them around.

There's no Saudi law specifically banning female drivers, but religious authorities have succeeded in enforcing a ban — much to the frustration of the kingdom's increasingly well-educated and professional female workforce.

Some women, however, see the new subway as offering them greater independence, and a way to save money.

"For sure I will use the metro — it will be a major solution for the women problem in our society, since we don't drive," Alaa Hassan, a female university student in Riyadh, told Reuters.

"I go to my university by minibus, and I pay 2,000 riyals ($535) per month; other classmates who live nearer pay 800 to 1,000. For sure the metro will be cheaper."

The Riyadh metro carriages will have special "family class" cars, which planners say will give women privacy similar to the "ladies only" carriages found in the New Delhi metro and others in Cairo, Tokyo andRio de Janeiro.

After two years of high oil prices, the Saudi government has a lot of cash, but low interest rates and a depressed world economy mean there's not much of a return on foreign investment.

That's one reason why the Saudis have seized the chance to beef up their infrastructure.

The government also wants to improve the daily lives of the poorer sections of its population of 28 million. Some analysts say that is an effort by the Saudi monarchy to avoid the kind of political protests that have led to popular revolutions elsewhere during the Arab Spring.